I saw my GF beat this game first, took her a few loops but she did and she had options for newgame+ and endless mode. Which one is the way to go? I am asking because I am coming up on my final payment (one loop so far!) and am unsure how to keep playing afterwards.

In newgame+, you start on day 2 again with pretty much everything you had -- items levels, hearts, dungeon stats. Money is brought back to 1000 and you have to expand the store and re-unlock adventurers You also have to unlock dungeons again, but previously unlocked floors are still accessible. At the start, you get to choose which savegame to restart from, and you can do this any point in time.

In endless mode, you don't have to worry about weekly payments anymore, so you cna do what you want, and the Lapis Ruins dungeon is unlocked (once you've beaten Obsidian Tower). This is a massive dungeon that can keep you busy for a long while. After that comes Crystal Nightmare, which certainly lives up to its name.

There's plenty of new stuff in endless mode, so maybe try that first for a bit. You can always go back to newgame+ when you feel like it. Newgame+ is mostly for if you want 100% completion, for which you need all the adventurers True Cards (you'll find out about that soon enough). Since you keep all your stuff in newgame+, you can probably make enough for the whole month in a week, so you'll be back in endless mode again soon enough anyway.